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Approximate reasoning for probabilistic real-time processes. Radha Jagadeesan DePaul University Vineet Gupta Google Inc Prakash Panangaden McGill University. Outline of talk. Beyond CTMCs to GSMPs The curse of real numbers Metrics Uniformities Approximate reasoning. - PowerPoint PPT Presentation
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Approximate reasoning for probabilistic real-time processes
Radha Jagadeesan DePaul University
Vineet Gupta Google Inc
Prakash Panangaden McGill University
Outline of talk
Beyond CTMCs to GSMPs The curse of real numbers Metrics Uniformities Approximate reasoning
Real-time probabilistic processes
Add clocks to Markov processes
Each clock runs down at fixed rate
Different clocks can have different rates
Generalized Semi Markov Processes: Probabilistic multi-rate timed automata
Generalized semi-Markov processes.
Each state is labelledwith propositional Information
Each state has a setof clocks associated with it.
{c,d}
{d,e} {c}
s
tu
Generalized semi-Markov processes.
Evolution determined bygeneralized states <state, clock-valuation>
<s,c=2, d=1>
Transition enabled when a clockbecomes zero
{c,d}
{d,e} {c}
s
tu
Generalized semi-Markov processes.
<s,c=2, d=1> Transition enabled in 1 time unit
<s,c=0.5,d=1> Transition enabled in 0.5 time unit
{c,d}
{d,e} {c}
s
tu
Clock c
Clock d
Generalized semi-Markov processes.
c. This need not be exponential.
{c,d}
{d,e} {c}
s
tu
Clock c
Clock d
0.2 0.8
Transition determines:
a. Probability distribution on next states
b. Probability distribution on clock values for new clocks
Generalized semi Markov processes If distributions are continuous and states are
finite:
Zeno traces have measure 0
Continuity results. If stochastic processes from <s, > converge to the stochastic process at <s, >
The traditional reasoning paradigm
Establishing equality: Coinduction Distinguishing states: HM-type logics Logic characterizes the equivalence (often
bisimulation) Compositional reasoning: ``bisimulation is
a congruence’’
Labelled Markov Processes
PCTL Bisimulation [Larsen-Skou,
Desharnais-Edalat-P]
Markov Decision Processes
Bisimulation [Givan-Dean-Grieg]
Labelled Concurrent Markov Chains
PCTL [Hansson-Johnsson]
Labelled Concurrent Markov chains (with tau)
PCTLCompleteness: [Desharnais-
Gupta-Jagadeesan-P]
Weak bisimulation [Philippou-Lee-Sokolsky,
Lynch-Segala]
With continuous timeContinuous time Markov chains
CSL [Aziz-Balarin-Brayton-
Sanwal-Singhal-S.Vincentelli]
Bisimulation,Lumpability
[Hillston, Baier-Katoen-
Hermanns,Desharnais-P]
Generalized Semi-Markov processes
Stochastic hybrid systems
CSL
Bisimulation:?????
Composition:?????
The curse of real numbers: instability
Vs
Vs
Problem!
Numbers viewed as coming with an error estimate.
Reasoning in continuous time and continuous space is often via discrete approximations.
Asking for trouble if we require exact match
Idea: Equivalence metrics
Jou-Smolka90, DGJP99, …
Replace equality of processes by (pseudo) metric distances between processes
Quantitative measurement of the distinction between processes.
Criteria on approximate reasoning
Soundness Usability Robustness
Criteria on metrics for approximate reasoning Soundness
Stability of distance under temporal evolution: “Nearby states stay close” through temporal evolution.
``Usability’’ criteria on metrics
Establishing closeness of states: Coinduction.
Distinguishing states: Real-valued modal logics.
Equational and logical views coincide: Metrics yield same distances as real-valued modal logics.
``Robustness’’ criterion on approximate reasoning The actual numerical values of the
metrics should not matter too much. Only the topology matters? Our results show that everything is defined
“up to uniformities.’’
What are uniformities?
In topology open sets capture an abstract notion of “nearness”: continuity, convergence, compactness, separation …
In a uniformity one axiomatises the notion of “almost an equivalence relation”: uniform continuity, …
Uniform continuity is not a topological invariant.
Uniformities: definition
A nonempty collection U of subsets of SxS such that:
Every member of U contains If X in U then so is If X in U, there is a Y s.t. YoY is contained
in X Down closed, intersection closed
Two apparently different Uniformities which are actually the same
m(x,y) = |x-y| m(x,y) = |2x + sinx -2y – siny|
Uniformities (different)
m(x,y) = |x-y|
Our results
Our results
A metric on GSMPs based on Wasserstein-Kantorovich and Skorohod
A real-valued modal logic Everything defined up to uniformity
Results for discrete time models
Bisimulation Metrics
Logic (P)CTL(*) Real-valued modal logic
Compositionality Congruence Non-expansivity
Proofs Coinduction Coinduction
Results for continuous time models
Bisimulation Metrics
Logic CSL Real-valued modal logic
Compositionality ??? ???
Proofs Coinduction Coinduction
Metrics for discrete time probabilistic processes
Defining metric: An attempt
Define functional F on metrics.
Metrics on probability measures
Wasserstein-Kantorovich
A way to lift distances from states to a distances on distributions of states.
Metrics on probability measures
Not up to uniformities
If the Wasserstein metric is scaled you get the same uniformity, but when you compute the fixed point you get a different uniformity because the lattice of uniformities has a different structure (glbs are different) then the lattice of metrics.
Variant definition that works up to uniformities
Fix c<1. Define functional F on metrics
Desired metric is maximum fixed point of F
Reasoning up to uniformities
For all c<1 we get same uniformity
[see Breugel/Mislove/Ouaknine/Worrell]
Metrics for real-time probabilistic processes
Generalized semi-Markov processes.
{c,d}
{d,e} {c}
s
tu
Clock c
Clock d
Evolution determined bygeneralized states <state, clock-valuation>
: Set of generalized states
The role of paths
In the continuous time case we cannot use single actions: there is no notion of “primitive step”
We have to talk about a “timed path” of one process matching a “timed path” of another process.
Generalized semi-Markov processes.
{c,d}
{d,e} {c}
s
tu
Clock c
Clock d
Path:
Traces((s,c)): Probability distribution on a set of paths.
Accomodating discontinuities: cadlag functions
(M,m) a pseudometric space. cadlag if:
Countably many jumps, finitely many jumpshigher than any fixed “h”.
Defining metric: An attempt
Define functional F on metrics. (c <1)
traces((s,c)), traces((t,d)) are distributions on sets of cadlag functions.
What is a metric on cadlag functions???
Metrics on cadlag functions
Not separable!
are at distance 1 for unequal x,y
x y
Skorohod’s metrics on cadlag
Skorohod defined 4 metrics on cadlag: J1,J2
M1 and M2 with different convergence
properties.
All these are based on “wiggling” the time.
The M metrics “fill in the jumps”.
The J metrics do not.
Skorohod metric (J2)
(M,m) a pseudometric space. f,g cadlag with range M.
Graph(f) = { (t,f(t)) | t \in R+}
t
fg
(t,f(t))
Skorohod J2 metric: Hausdorff distance between graphs of f,g
f(t)g(t)
Skorohod J2 metric
(M,m) a pseudometric space. f,g cadlag
Examples of convergence to
Example of convergence
1/2
Example of convergence
1/2
Examples of convergence
1/2
Examples of convergence
1/2
Non-convergence in J2:
Sequences of continuous functions cannot converge toa discontinuous function.
In general, the number of jumps can decrease in the limit,but they cannot increase.
Non-convergence
Non-convergence
Non-convergence
Non-convergence
Summary of Skorohod J2
A separable metric space on cadlag functions
Allows jumps to be nearby Allows jumps to decrease in the limit. Not complete.
Defining metric coinductively
Define functional on 1-bounded pseudometrics (c <1)
Desired metric: maximum fixpoint of F
a. s, t agree on all propositions
b.
Results
All c<1 yield the same uniformity. Thus, construction can be carried out in lattice of uniformities.
Real valued modal logic which gives an alternate definition of a metric.
For each c<1, modal logic yields the same uniformity but not the same metric.
Proof steps
Continuity theorems (Whitt) of GSMPs yield separable basis.
Finite separability arguments yield the result that the closure ordinal of the functional F is omega.
Duality theory of LP for calculating metric distances.
Summary
Metric on GSMPs defined up to uniformity. Real valued modal logic that gives the
same uniformity. Approximating quantitative observables:
Expectations of continuous functions are continuous.
Might be worth looking at the M2 metric.
Real-valued modal logic
Real-valued modal logic
Real-valued modal logic
Real-valued modal logic
h: Lipschitz operator on unit interval
Real-valued modal logic
Base case for path formulas??
Base case for path formulas
First attempt:
Evaluate state formula F on stateat time t
Problem: Not smooth enough wrt time sincepaths have discontinuities
Base case for path formulas
Next attempt:
``Time-smooth’’ evaluation of state formula F at time t on path
Upper Lipschitz approximation to evaluatedat t
Real-valued modal logic
Non-convergence
Illustrating Non-convergence
1/2
1/2